The vast majority of transport aircraft employ pylons to attach turbofan or turboprop engines beneath the wing of the aircraft. A channel is formed by the side of the aircraft's fuselage, the under surface of the wing between the fuselage and the pylon, the inboard surface of the pylon, and the engine's nacelle. The aerodynamic shapes of the elements forming this channel greatly influence the airflow through it.
Due to structural considerations, a typical pylon has a chord length equal to the local chord length of the wing section to which it is attached with its maximum thickness occurring between 20 and 40 percent of the chord length of the pylon. To streamline the pylon, the cross-sectional area is progressively increased from the leading edge of the pylon to its thickest point in a curved fashion. This curvature is evenly distributed on each side of the pylon centerline. The cross-sectional area of the pylon is then progressively reduced toward the pylon trailing edge in a similar curved, symmetrical fashion.
The fuselage-wing-pylon-nacelle channel using the above described pylon causes a supersonic airflow phenomenon called wave drag, along with other aerodynamic problems, in a typical subsonic transport aircraft. The cross-sectional area of the channel decreases as the cross-sectional area of the pylon increases. The airflow through this channel accelerates until it reaches a velocity equal to the local speed of sound at the minimum cross-sectional channel area, at the maximum thickness position of the pylon. A normal shock wave occurs at this point. The flow velocity continues to increase past Mach 1 as the channel area increases due to the decrease in pylon thickness. This sonic and supersonic flow results in (1) loss of lift due to a decrease in static pressure under the wing, (2) an increase in form drag due to flow separation, and (3) wave drag due to the shock waves that are produced.
Past attempts to alleviate these unfavorable effects have not been totally successful. For example, one method is to flatten the inboard side of the pylon to reduce the local flow velocities. Unfortunately, since the pylon thickness must remain the same, the curvature on the outboard side must increase. This causes increased pylon flow separation and pylon drag.
Another method is to shape the pylon to conform to streamlines immediately adjacent to the pylon, as disclosed in U.S. Pat. No. 4,449,683 by Gratzer et al. The pylon has a less retarding effect on the span flow under the wing, so the induced drag of the airplane is increased over conventional pylons.
Yet another method is to modify a conventional pylon by either mounting a broad fairing on its inboard surface, or placing a tapered bump at the inboard intersection between the wing and pylon. Both embodiments are disclosed in U.S. Pat. No. 4,314,681 by Kutney. They effectively reduce the channel flow area, preventing supersonic airflow. However, the relatively crude addition of material to the pylon increases other forms of drag.